1. Motivation. Why study ground state hyperon electroproduction? CLAS detector and analysis. Analysis results. Current status and future work. M. Gabrielyan 1, B. Raue 1, D.S. Carman 2, K. Park 2 1.Florida International University 2.Jefferson Lab NSTAR 2011 5/19/2011 Measurement of the Induced Λ (1116) Polarization in K + Electroproduction at CLAS

2. Motivation This study is part of a larger program that has a goal of measuring as many observables as possible for KY electroproduction.  Understand which N*’s couple to KY final states.  These data are needed in a coupled-channel analysis to identify previously unobserved N* resonances.  Get a better understanding of the strange-quark production process by mapping out the kinematic dependencies for these observables.  The results will tell us which (if any) of the currently available models best describe the data.

3. CEBAF Large Acceptance Spectrometer 3 regions of drift chambers located spherically around target provide charged particle tracking for angle and momentum reconstruction Toroidal magnetic field in region 2 Cherenkov detectors provide e/  separationCherenkov detectors provide e/  separation Time of flight scintillators for particle ID Electromagnetic calorimeters give total energy measurement for electrons and neutrals and also e/  separationElectromagnetic calorimeters give total energy measurement for electrons and neutrals and also e/  separation

4. Kinematics and E1F Dataset Beam energy = 5.5 GeV Unpolarized Target Torus current = 2250 A 5B triggers, 213000 Λ ’s 0.8 < Q 2 < 3.5 GeV 2 1.6 < W < 2.8 GeV -1.0 < cos( θ K CM ) < 1.0 Q 2 vs WQ 2 vs cos( θ K CM )

5. Particle Identification Electrons:  Coincidence between CC and EC in the same sector.  Negatively charged track in DC that matches in time with TOF.  Momentum corrections applied to correct for DC misalignments and inaccuracies in the magnetic field map. Hadrons: Time difference (Δt) between the measured time and the computed time for a given hadron species (  , K +, p). Minimum Δt identifies the hadron.

6. Hadron Identification Minimum Δt identifies the hadron. Minimum Δ t vs p After Λ and  missing mass cuts pK+K+

7. Hadron Identification Minimum Δt identifies the hadron. Minimum Δ t vs p  given K mass p given K mass After Λ and  missing mass cuts pK+K+

8. Λ Identification Background in the hyperon missing mass spectrum is dominated by  ’s misidentified as K +. Λ(1116) Σ(1192) MM 2 (π - ) vs MM(eK + ) MM(eK + ) MM 2 (π - )  Reconstructed missing mass for e+p  e’K + (Y)  For recoil polarization observables e+p  e’K + p  -  include  - missing-mass cut

9. Cross Section for Electroproduction Polarized beam & recoil Λ, unpolarized target. Where: Induced polarization Transferred polarization

10. Λ Polarization Extraction Parity non-conservation in weak decay allows to extract recoil polarization from p angular distribution in Λ rest frame. where: α=0.642 ± 0.013 (PDG) Here N F and N B are the acceptance corrected yields. After Φ integration only P N component survives for induced polarization (P L, P T = 0). Carman et al., PRC 79 065205 (2009)

11. Acceptance Corrections FSGen: Phase space generator with modified t-slope : t-slope = 0.3 GeV -2 Acceptance corrections are applied to background subtracted yields. MC DATA Θ K vs φ K CM Acceptance factors vs W 0.8<Cos( θ K CM )< 1.0 W T N L

12. Background Subtraction The fit function form is motivated by the Λ and Σ Monte Carlo templates that are matched to data.  f Λ = G Λ + L Λ L + L Λ R  f Σ = G Σ + L Σ L + L Σ R  f BKG = A*(bkg_temp)  f TOTAL = f Λ + f Σ + f BKG G Λ = Gaussian, L Λ L = Left Lorentzian, L Λ R = Right Lorentzian, A = Amplitude. The background templates are generated from data by intentionally misidentifying pions as kaons. G Λ + L Λ L G Λ + L Λ R

13. Background Subtraction FWD χ 2 =1.003 T: W=2.575 N: W =2.0125 Λ0Λ0 ∑0∑0 Bkg. Total T: W=1.7375 N: W =2.0125 BKWD FWD χ 2 =1.063 FWD χ 2 =1.075 MM(eK + ) Cos( θ K CM )=0.9 1.05 < MM(eK + ) < 1.15 GeV

14. Preliminary Results SUM over Q 2, Φ Systematics Check: P L vs W W -1.0<Cos( θ K CM )<-0.5 -0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2 0.2<Cos( θ K CM )<0.40.8<Cos( θ K CM )< 1.0 0.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8 W

15. Induced Polarization P N vs W Preliminary Results SUM over Q 2, Φ ≤ 0.06 -1.0<Cos( θ K CM )<-0.5 -0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2 0.2<Cos( θ K CM )<0.4 0.8<Cos( θ K CM )< 1.0 0.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8 WW

16. RPR Model Non-resonant background contributions treated as exchanges of kaonic Regge trajectories in the t-channel: K(494) and K * (892) dominant trajectories. Both have a rotating Regge phase. This approach reduces the number of parameters. Included established s-channel nucleon resonances: S11(1650), P11(1710), P13(1720), P13(1900) Included missing resonance: D13(1900). Model was fit to forward angle (cos θ K CM > 0) photoproduction data (CLAS, LEPS, GRAAL) to constrain the parameters. Corthals et al., Phys. Lett. B 656 (2007)

17. Induced Polarization P N vs W Preliminary Results SUM over Q 2, Φ -1.0<Cos( θ K CM )<-0.5 -0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2 0.2<Cos( θ K CM )<0.40.8<Cos( θ K CM )< 1.0 0.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8 WW

18. Induced Polarization P N vs W Preliminary Results SUM over Q 2, Φ -1.0<Cos( θ K CM )<-0.5 -0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2 0.2<Cos( θ K CM )<0.40.8<Cos( θ K CM )< 1.0 0.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8 WW

19. Induced Polarization vs W (photoproduction) Red: McCracken, CLAS 2010 Blue: McNabb, CLAS 2004 Green: Glander, SAPHIR 2004 Black: Lleres, GRAAL 2007 Dashed lines indicate the physical limits of polarization. W ( GeV) M.E. McCracken et al., Phys. Rev. C 81, 025201 (2010).

20. Induced Polarization vs W (photoproduction) M.E. McCracken et al., Phys. Rev. C 81, 025201 (2010). Red: McCracken, CLAS 2010 Blue: McNabb, CLAS 2004 Green: Glander, SAPHIR 2004 Black: Lleres, GRAAL 2007 Dashed lines indicate the physical limits of polarization. W ( GeV) -0.5<Cos( θ K CM )< 0.0 0.8<Cos( θ K CM )< 1.0

21.  Background subtraction and acceptance corrections are complete.  RPR theoretical model calculations are in good agreement with experimental data at very forward kaon angles but they fail to reproduce the data at all other kaon angle bins. RPR gives a reasonable description of photoproduction data (cos θ K CM > 0).  Experimental data are similar for both electro- and photoproduction at forward kaon angles, but are very different for backward kaon angles. NEXT… Complete the systematic error analysis. Comparison to different theoretical models. Funded in part by: The U.S. Dept. of Energy, FIU Graduate School SUMMARY

22. Systematics Check: P T vs W Preliminary Results SUM over Q 2, Φ -1.0<Cos( θ K CM )<-0.5 -0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2 0.2<Cos( θ K CM )<0.40.8<Cos( θ K CM )< 1.0 0.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8 WW

23. Polarization vs Q 2, Sum over Cos( θ K CM ), Φ 1.6<w<1.625 GeV1.625<w<1.65 GeV 1.65<w<1.675 GeV1.675<w<1.7 GeV 1.7<w<1.725 GeV1.725<w<1.75 GeV 1.75<w<1.775 GeV1.775<w<1.8 GeV 1.8<w<1.825 GeV1.825<w<1.85 GeV 1.85<w<1.875 GeV1.875<w<1.9 GeV

24. Acceptance Factors vs W -1.0<Cos( θ K CM )<-0.5-0.5<Cos( θ K CM )<0.0 0.0<Cos( θ K CM )<0.2

25. Acceptance Factors vs W 0.2<Cos( θ K CM )<0.40.4<Cos( θ K CM )<0.6 0.6<Cos( θ K CM )<0.8